The present invention relates to devices for measuring, in operation, the non-linearity product levels in equipment for the transmission and/or retransmission of television signals and, in particular, the power stages for such equipment.
The non-linearity of equipment in general seriously deteriorates the quality of the transmitted signals, particularly when the equipment transmits several carriers, one of which is amplitude-modulated in a wide band, as is the case in television.
As a result of this non-linearity interference frequencies are produced as a result of the beat of the initial frequencies and/or their harmonics, which is generally called intermodulation, and there is an amplitude modulation of each initial frequency, the value of said amplitude being a function of the amplitude of the other frequencies constituting cross modulation.
Outside the operating periods many devices and instruments exist enabling technicians to maintain and regulate the equipment so as to reduce such phenomena and make them compatible with the quality needed for the transmitted signals. However, it is often desirable to monitor the transmission quality during operation.
For this purpose it is known to use test signals included in one of the lines of the field blanking moments for the duration of which an intermodulation measurement is performed in the band transmitted, for example, at the frequency F.sub.I +F.sub.S -F.sub.BL in which F.sub.I and F.sub.S are vision and sound carrier frequencies and F.sub.BL the frequency of a side band. This measurement represents an artificial, fixed state of the equipment load and in particular is not always available. In fact in certain countries there is no signal at the test frequency.